If you filled the gas tank to the brim then parked it in the sun, the gas will expand in the heat and it has to go somethere.
If that is not the case, if you know the tank is not completely full, then you need to look for the leak.
Is this a California bike maybe? They have more venting/overflow hardware than bikes sold in the rest of the US. Its also possible you have a loose hose clamp somewhere on a fuel hose, and its just dribbling down the overflow tube.
If your are sure its coming out the overflow tube, if you can see it coming out, pop the gas cap and see if it stops? If it does then your tank vent is plugged up.